As wtallis said: Cortex-M executes only Thumb instructions, while pre-ARM7TDMI don't execute Thumb instructions.
Cortex-A, Cortex-M, Cortex-R have different targets, ISA, System organization as far back as their origination. Not sure why you'd compare them, I was obviously talking about processors within the same segment, Cortex-R7 is backwards compatible with previous versions, Cortex-A72 is backwards compatible with previous versions, Cortex-M3 is backwards compatible with previous versions.
AFAIK, ARMv7 in user/system mode is backwards compatible with ARMv5 at-least. Even AARCH32 claims so.